Case of the Faulty Oscillator

So, in spite of my apprehension, I asked that we look at the power supply again. Reluctantly but respectfully, my colleagues obliged. They connected a digital oscilloscope to the oscillator power supply, and they were right. It was a nice, flat DC supply. They increased the vertical gain, and we measured the ripple, which was practically nonexistent.

Then I noticed that the sweep time on the scope was quite long. I shortened the sweep time, incrementally searching for any AC anomalies riding on the oscillator power supply. Lo and behold, a 2Vpp sine wave showed up looking quite anomalous by taking up most of the oscilloscope's display.

Where was that coming from? We measured its frequency and found that it matched the spectral separation between the oscillator's carrier and each sideband exactly! The telltale 2Vpp signal was traced back to an IC regulator that was oscillating and superimposing its instability on our "flat" DC.

We had the problem on the run. While checking the regulator installation, we found that the newly installed regulator IC was, in fact, the wrong part, and, more importantly, it had been replaced a few days earlier. As usual, hindsight is 20/20. If I had originally checked to see which components had been replaced, I would have put two and two together as soon as I suspected the power supply.

This entry was submitted by John Mitchell and edited by Rob Spiegel.

Tom Machell is an electronics engineer who has worked for Northrop Grumman and L3 Communications. He is presently an avionics/electrical engineer at IMP Aerospace in Halifax, Nova Scotia.

How true! Turns out the main reason why I didn't look closer the first time was because I was working with techies who already assured me that the DC was fine. I had originally suspected the power supply but assumed that they had checked it ok. Never assume anything.

Ha...! Yeah...I know what you mean. I had one where a techie was testing an inductor using the ringing test on a Sencore RCL meter and found that all the devices were failing. Turns out that the manual stated that the ringing test was not appropriate to test inductors of that value.

Another one was where someone had spec'd an intrinsically safe milliohm meter to be used in fuel tanks. Turns out that the accuracy wasn't good enough to make the application measurement. He commented on the fact that he hadn't considered the accuracy specs prior to recommending the meter. Wha?? It all worked out in the end. The nota bona here is that no one is perfect, including me. This is why all aircraft maintenance requires that there be an independent inspection post repair.

Reminds me of another coworker who was trying to get an inverter chip to work and became really annoyed that he couldn't get any output. He fought with it most of that day only to realize that he had grounded the chip...boy did he feel foolish...to this day, he and I still use "did you ground the chip?" question to poke fun when either of us does something stupid or misses the obvious. It happens.

The first thing that came to mind is, if there was a lower frequency waveform on the power supply line, why couldn't they see it even at a short time base? One end or the other of the wave would have wandered from baseline. Is it because they were using a digital scope? One of the persistent problems I see is that digital scopes do not represent noise correctly. You either don't see it or it looks horrendous but isn't really there. They need a good combo scope, oh, wait, you mean almost nobody makes them anymore???

Then that doesn't say much for that very expensive digital scope. It was definitely mis-leading them. For the life of me, I can't figure out how you can have 2V pp of noise and not see it. How can you have that amount of noise and see, on your very expensive digital scope, a dead flat line??? Case in point, we had one customer complaining about a 25 hz oscillation coming out of our system. It's an eddy-current displacement system and the only frequency normally on this system is the 500khz to the sensor. As it turned out, their (also very expensive) digital scope was aliasing the residual 500khz carrier. As they changed the time base, the frequency of the 'noise' changed until they got to 500khz.

As long as you really understand how to use them. Knowing their weaknesses, an analog 'scope will always have a place on my bench.

Any instrument can "lie" to you if you're not careful about how you set it up for the measurement. It also pays to ensure the instrument is appropriate for the measurement you are trying to make. This point was made earlier on but bears repeating.

IT is easy to get results that lead to incorrect conclusions when measuring with a scope. The challenge with digital scopes is that they can be misused in many more ways. It is not a fault of the scope, except for the ones that don't allow an adequate amount of bandwidth reduction.Sometimes it is very handy to nt have the 20MHZ bandwidth displaying all of the ambient noise, when what I want to see is the ten KHZ noise from a system. It is really hard to read the value of a signal that has an eighth-inch thick fuzzy trace of 17MHz noise from the ambient, when I know that the noise I want to read accurately is in the hundresds of Hz.

But with all of the features, it can be challenging to even understand how to use some digital scopes. That certainly does lead to problems and incorrect readings.

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